Photoelectrophoretic imaging system

ABSTRACT

A photoelectrophoretic imaging system is disclosed wherein a thermoconductive material such as polyethylene is used as the blocking electrode of the system. The thermoconductor is heated to discharge static electricity accumulated on its surface and cooled to restore its high insulating property. The heating and cooling is accomplished by using heated rollers, gas streams and infrared radiation.

United States Patent 1191 Gundlach 1 Dec. 4, 1973 PHOTOELECTROPHORETICIMAGING 3,535,221 10/1970 Tulagin 96/13 x SYSTEM 3,676,313 7/1972Ciccarelli.

. 3,595,771 7/1971 Weigl 96/12 X [75] Inventor: Robert W. Gundlach,Victor, NY.

[73] Assigneez Xerox Corporation, Rochester, NY. Primary Examiner RobenP Greiner [22] Filed: Oct. 15, 1971 AttorneyJames J. Ralabate et a1.

[21] Appl. No.: 189,729

Related US. Application Data [62] Division of Ser. No. 829,624, June 2,1969, Pat. No.

[52] US. Cl 355/3, 96/13, 355/4 [51] Int. Cl G03g 15/00 [58] Field ofSearch 355/3, 4, l7;

[56] References Cited UNITED STATES PATENTS 3,586,615 6/1971 Carreira96/].3 X

cooling is accomplished by using heated rollers, gas

streams and infrared'radiation.

1 Claim, 4 Drawing Figures 1 PHOTOELECTROPHORETIC IMAGING SYSTEM This isa division, of application, Ser. No. 829,624, filed June 2, 1969 now US.Pat. No. 3,639,224.

BACKGROUND OF THE INVENTION This invention relates in general to imagingsystems. More specifically, the invention concerns an improvedphotoelectrophoretic imaging system.

There has beenrecently developed an elecrophoretic imaging systemcapable of producing color images which utilizes electricallyphotosensitive particles. This process is described in detail andclaimed in US. Pat. Nos. 3,384,566 to H. E. Clark, 3,384,565 to V.Tulagin et al., and 3,383,993 to Shu-Hsiung Yeh. In such an imagingsystem, variously colored light absorbing particles are suspended in anon-conducting liquid carrier. The suspension is placed betweenelectrodes, one of which is generally conductive, called the injectingelectrode and the other of which is generally insulating and called theblocking electrode. One of these electrodes is at least partiallytransparent to activating electromagnetic radiation. The suspension issubjected to a potential difference between the electrodes across thesuspension and exposed to an image through said transparent electrode.As these steps are completed, selective particle migration takes placein image configuration, providing a visible image at one or both of theelectrodes. An essential component of the system is the suspendedparticles which must be electrically photosensitive and which apparentlyundergo a net change in charge polarity upon exposure to activatingelectromagnetic radiation when within interaction range of one of theelectrodes. In a monochromatic system, particles of a single color areused, producing a single colored image equivalent to conventionalblack-and-white photography. in a polychromatic system, the images areproduced in natural color because mixtures of particles of two or moredifferent colors which are each sensitive to light of a specificwavelength or narrow range of wavelengths are used.

This system, using a conductive injecting electrode, a substantiallyinsulating blocking electrode and photosensitive particles dispersed inan insulating carrier liquid between the electrodes has been found to becapable of producing excellent images The insulatimg properties of theblocking electrode have been found to affect the quality of imagesproduced by the system. Materials having a volume resistivity in theorder of l to ohm-centimeters (cms) have produced satisfactory imagesbut improved image quality is seen when the resistivity of the blockingelectrode is in the order of 1O ohm-ems or greater. However, thematerials having the 10 ohm-cms resistivity have not proved successfulin cycling imaging situations because of the accumulation of undesirablecharge on their surfaces. The reason for this may be understood by aconsideration of the time constant 1 in seconds for the discharge ofstatic electricity from an insulator having a dielectric constant K andresistivity p in ohm-ems, which may be calculated to be 1-=8.85 X 10 K pwhere the numerical constant has the units seconds ems/ohm. Practicalblocking electrodes have dielectric constants between 2 and 200 or more,typically about 10. For a resistivity of p-lO ohm-ems therefore the timeconstant for 63 percent discharge of accumulated static is 0.885 K, orabout 1.8 to 180 seconds, typically about 9 seconds.

For p-lO ohm-ems these times per ten times longer. Since commercialmachines must frequently cycle every 0.1 to I second, it is necessary todischarge the blocking layer at least this rapidly between cycles. It isnecessary to provide electrode layers which are blocking, and thereforeinsulating, at the imaging station, but which may be renderedtemporarily sufficiently conductive, with, e.g., p less than about 10ohm-cms and preferably 10" l0 ohm-cms to allow rapid discharge betweencycles.

One solution to the above problem is to-use a photoconductive materialat the blocking electrode. Following the imaging step the photoconductoris separated from the imaging suspension and flooded with light torender it conductive and thereby bleed the charge from its surface. Thistechnique, although highly successful, has an inherent limitation inthat the photoconductor is also subjected to light during the imagingstep. This limitation is not always severe because the imagingsuspension shields the photoconductor at least partially during theimaging step. Also, photoconductive materials are available which areeither not sensitive to the imaging light or are much less sensitive tothe imaging light than the imaging suspension. Nonetheless, it isdesirable to devise a highly insulating blocking electrode that can berapidly discharged in response to interaction with a medium other thanlight because the imaging suspension is also sensitive to light.Practical engineering considerations suggest the restriction of thequantity of light used in the system. Furthermore, the presently knownmaterials available for use as blocking electrodes do not all have idealcharacteristics in the areas of humidity resistance, cleanability,surface smoothness, abrsion resistance and cost. Consequently, there isa continuing need for improved blocking electrode materials and formethods of improving the performance of these materials in thephotoelectrophoretic process.

It is therefore an object of this invention to increse the speed atwhich successive images can be formed by the photoelectrophoreticimaging process. Also, it is an object of the invention to overcome theabove mentioned disadvantages.

Still another object of the invention is to broaden the range ofmaterials available for the blocking electrodes of aphotoelectrophoretic imaging system by using novel means in thephotoelectrophoretic system for varying the resistivity of the blockingelectrode material.

A further object of the invention is to devise means for dissipatingcharge accumulated-on the surface of the blocking electrode withoutinterfering with the steps of the photoelectrophoretic process.

The foregoing and other objects of the present invention are attained byemploying a thermoconductive material on the blocking electrode of aphotoelectrophoretic system. A thermoconductive material is a materialwhose resistivity varies with temperature. In the present invention thethermoconductive material is insulating, i.e. has a high resistance, atthe imaging step of the photoelectrophoretic process, is subsequentlyremoved from the imaging station and heated to reduce its resistance,thereby enabling residual charge on its surface to be rapidlydischarged. Thereafter the thermoconductor is cooled to restore its highresistance and a subsequent imaging operation is initiated. The presentinvention, therefore, provides means for rap- 3 idly discharging staticelectricity accumulated on the surface of a blocking electrode byeffecting heat transfers with the surface of the electrode. In addition,because the present invention is not dependent upon light for itsoperation, it has the distinct advantage over a system using aphotoconductor blocking electrode in that it eliminates the possibilityof 'a discharging light source interfering with the imaging operationand of the imaging light source interfering with the blocking electrodeoperation.

Any suitable temperature sensitive material may be used as thethermoconductive blocking electrode. It may be homogeneous or a mixtureof two or more materials and may be organic or inorganic. In addition,it may comprise heat sensitive particles dispersed in a heat resistantbinder. The presently preferred material for use as the thermoconductiveblocking electrode is polyethylene, since it has the dielectric strengthto withstand the voltages employed in the photoelectrophoretic processand because it has a sharp decrease in resistance at temperatures towhich it is readily heated and from which it is readily cooled.

DESCRIPTION OF THE DRAWINGS The advantages of this improved method ofimaging will become apparent upon consideration of the detaileddisclosure of the invention especially when taken in conjunction withthe accompanying drawings wherein:

FIG. 1 is a schematic side-sectional view of a simplephotoelectrophoretic imaging system utilizing a thermoconductiveblocking electrode with conduction rollers in contact therewith forheating and cooling the thermoconductor;

FIG. 2 is a partial view of the system in FIG. 1 illustrating anotherembodiment of the present invention wherein the heating and cooling ofthe thermoconductive blocking electrode is accomplished by directingstreams of gas onto the blocking electrode;

FIG. 3 is a partial view of the system in FIG. 1 illustrating anotherembodiment of the invention wherein an infrared radiation source is usedto heat the surface of the thermoconductive blocking electrode; and

FIG. 4 is a graph of resistance vs. temperature for polyethylene.

DESCRIPTION OF THE DRAWINGS Referring now to FIG. I there is seen atransparent electrode generally designated 1 which in this exemplaryinstance is made up of a layer of optically transparent glass 2overcoated with a thin optically transparent layer 3 of tin oxidecommercially available under the name NESA glass. This electrode willhereafter be referred to as the injecting electrode. Coated on thesurface of injecting electrode 1 is a thin layer 4 of finely dividedelectrically photosensitive particles dispersed in an insulating liquidcarrier. Electrically photosensitive for the purposes of this disclosurerefers to the properties of a particle which, once attracted near theinjecting electrode, will migrate away from it under the influence of anapplied electric field when it is exposed to actinic electromagneticradiation. For a detailed theoretical explanation of the apparentmechanism of operation of the imaging process, see US. Pat. No.3,384,565 issued May 21, 1968 to V. Tulagin et al. and US. Pat. No.3,384,566 issued May 21, 1968 to H. E.

Clark, the disclosures of which are incorporated herein by reference.

Adjacent to liquid suspension 4 is a second electrode 5, hereinaftercalled the blocking electrode which is connected to one side ofpotential source 6 through switch 7. The opposite side of potentialsource 6 is connected to the injecting electrode 1 so that when switch 7is closed an electric field is applied across the liquid suspension 4between electrodes 1 and 5. An image projector made up of a light source8, a transparency 9, and a lens 10 is provided to expose the dispersion4 to a light image of the original transparency 9 to be reproduced.Electrode 5 is made in the form of a roller having a conductive centralcore or substrate 11 connected to potential source 6. Core 11 is coveredwith a layer of a thermoconductive insulating material 12 which may beany suitable thermoconductive insulating material as discussed furtherbelow. The particle suspension is exposed to the image to be reproducedwhile a potential is applied across electrodes 1 and 5 by closing switch7. Roller 5 is caused to roll across the top surface of injectingelectrode 1 with switch 7 closed during the period of image exposure.This light exposure causes exposed pigment particles originallyattracted to electrode 1 to migrate through the liquid and adhere to thesurface of blocking electrode 5, leaving behind a particulate image onthe injecting electrode surface which is a duplicate of the originaltransparency 9. After exposure, the relatively volatile carrier liquidevaporates off leaving behind the particulate image. This particulateimage may then be fixed in place, as for example, by placing alamination over its top surface or by virtue of a dissolved bindermaterial in the carrier liquid such as paraffin wax or other suitablebinder that comes out of the solution as the carrier liquid evaporates.In an alternative, the particulate image remaining on the injectingelectrode may be transferred to another surface and fixed thereon. Thissystem can produce either monochromatic or polychromatic imagesdepending upon the type and number of pigments suspended in the carrierliquid and the color of light to which the suspension is exposed in theprocess.

Where the above imaging steps are repeated, with cleaning of theblocking electrode and reapplication of the particle suspension onto theinjecting electrode but without discharging the blocking electrodebetween imaging operations, it has been found that there is a steadydecrease in the image quality in successive copies. It has been foundthat this gradual decrease in image quality is due to the accumulationof undesired electrostatic charge on the surface of the blockingelectrode. Therefore in accordance with this invention, before or afterthe blocking electrode has been cleaned of unwanted particles,thermoconductive layer 12 is heated to lower its resistivity and therebyto rapidly discharge accumulated charge through layer 12 to central core11.

The roller blocking electrode configuration shown in the drawings are ofcourse merely representative and any other suitable configuration may beused. Typically, the blocking electrode is comprised of athermoconductive layer mounted on a conductive substrate and may be inthe form of a moveable or stationary flat plate, or in the form of abelt entrained over rollers.

The presently preferred material for the thermoconductive layer 12 ismedium density polyethylene which has an electrical volume resistivityin the order of ohm-ems at 23 C and 50 percent relative humidity and adielectric constant in the order of 2.3. The decay time, T 0.885 PK 10*,required to dissipate charge on its surface is therefore in the order of1,840 seconds. Now in accordance with the present invention, lowertingthe resistivity of the polyethylene down to 10 or l0 ohm-ems or lowerreduces the decay time T to generally 0.l8 to 1.8 seconds which is wellwithin the cycling period of a high speed imaging system.

The curve in FIG. 4 is a plot of volume resistivity vs. temperature forpolyethylene coated paper. The curve illustrates that a significantchange in the resistivity of polyethylene is obtainable by heating andcooling the material. Consequently, the heating and cooling of theblocking electrode thermoconductor enables the best of two situations tobe obtained. Heating the thermoconductive materials lowers itsresistivity thereby enabling surface static electricity to be rapidlydischarged. Cooling the thermoconductor restores its high resistancepermitting optimum performance during the imaging operation. Thespecific temperature between which the thermoconductive material israised and lowered depends upon, among other parameters, the particularthermoconductive material used and the duration of the cycle time atwhich the imaging system operates. Obviously, the preferred temperaturesare selected for optimum performance for a given imaging system.

Generally, most materials experience a variation in resistivity withvariations in temperature. Amongst these class of materials arephotoconductors and as such they may be employed in the mannerprescribed by the present invention. A material available from theDuPont Corporation under the name Tedlar PVF film is another example ofa material having electrical and thermal characteristics suitable foruse as a blocking electrode in acccordance with the present invention.TedlarPVF film has a volume resistivity of 3 X l0 ohm-cms at 23 C and of1 X l0 ohm-cms at 132 C. Some materials experience an increase inresistivity with an increase in temperature which is the inverse of whathappens to polyethylene. If these materials have the desired dielectricconstants and resistivities they can be used in accordance with thepresent invention. In this case however, the material would be heated torender it sufficiently resistive for the imaging step and cooled tobleed the charge from its surface.

The thermoconductive layer 12 shown in FIGS. 1, 2, and 3 is relativelythin (approximately one thirtysecond inch) and therefore is a mass whichis rapidly heated and cooledwithin a short period of time. Consequently,variations in the resistivity of the blocking electrode can be madewithin a time period consistent with the cycle period of a high speedimaging system.

In FIG. 1, the heating and cooling of the thermoconductive layer 12 isaccomplished by the conduction rollers 16 and 17. The peripheries of theconduction rollers are in contact with the thermoconductor 12 to heatand cool the surface by conduction through the rollers. The hot roller16 has an outer layer 18 of steel (or other material that is a goodconductor of heat) and a heating element 19 within the core of layer 18.The heating element 19 includes an electrical heating coil but can beany other suitable heating element such as those using hot gasses orliquids. The cold roller 17 also has an outer layer 21 of steel (orother material that is a good conductor of heat) and a refrigerationelement 22 within the core of layer 21. The refrigeration element hascoiled conduits through which cooled gases or liquids are circulated inorder to lower the temperature of the cold roller. Because the outerlayer 18 of the hot roller 16 is also electrically conductive, it iscoupled to a ground potential by means of the conductive lead 23. Somecharge therefore may be bled from the surface of the thermoconductivelayer 12 by virtue of the contact between the roller 16 and thethermoconductor.

In FIG. 2, the heating and cooling of thermoconductive layer 12 isaccomplished by means of the nozzles 25 and 26 which direct streams ofhot and cold gas onto the surface of the thermoconductive layer 12. Thehot nozzle 25 is coupled to a source of pressurized gas 27, such ascompressed air, by the coiled conduit 28. The air in conduit 28 isheated by the electrical heating element 29 positioned within the coilsof conduit 28. Similarly, the cold nozzle 26 is coupled by coiledconduit 31 to the gas source 27. The air in conduit 31 is cooled by therefrigeration element 32 positioned within the coils of conduit 31. Thehot and cold gas streams emitted by the nozzles therefore heat and coolthe surface of the thermoconductor by conduction and convection. Each ofthe noules has a diaphragm adjacent the exit orifice to permit the airstream to be turned on and off.

In FIG. 3, the thermoconductor is again cooled by the cold nozzle 26 butin this case is heated by the infrared lamp 33. The rays of lamp 33 arefocused to a narrow area across the width of thermoconductive layer bymeans of a condensing lens 34.

The power units for driving the various heating and cooling elementssuch as the electrical heating coils 19, 29 and 31, the infrared lamp 33and the cooling or refrigeration coils 22 and 32 are of conventionaldesign and are not illustrated in the figures in order to simplify andthereby clarify the description.

In each of the above embodiments of the present invention, the majorityof the heat transfer with the thermoconductive layer 12 takes place overa narrow area across its width. The remaining surface area of thethermoconductor is subjected to the heat transfers by tuming the rollerthrough 360. Proceeding in this manner permits relatively small amountsof energy to be used for effecting the heat transfers by minimizing thethermal mass involved in the thermal cycling and permits the heattransfers to occur within periods of time consistent with the operationcycle of a high speed imaging system. Furthermore, controlling thethickness of the thermo-conductive layer and limiting the area overwhich the layer is heated increases the effectiveness of the apparatusused to cool the thermoconductive layer and can eliminate the need forsuch apparatus as cold roller 16 and cold nozzle 26 in certain systemconfigurations. The reason is that the heat transfer to thethermoconductor is over a relatively'small portion of its total surfacearea and accordingly is capable of being rapidly dissipated by radiationand conduction to the air and to the core of the roller.

The specific quantities of heat required to be transferred to and fromthe thermoconductor depends upon the characteristics of a selectedthermoconductive material and the system in which it is used. Enoughheat must be transferred to the thermoconductive layer to raise thetemperature to a level at which charge is rapidly dissipated. Likewise,enough heat must be transferred from the therrnoconductor or lower thetemperature to a level at which it is highly insulating. The specifictemperatures between which various materials must be varied to obtainthe desired results will depend on the values of resistivity as afunction of temperature, and the area over which it is heated and cooledat any point in time.

What is claimed is:

l. A photoelectrophoretic imaging apparatus comprising a first electrodeincluding a thennoconductive insulating layer, a second electrode, meansto bring said first electrode into and out of virtual contact with saidsecond electrode,

means to impose an electric field between said electrodes,

exposure means for exposing a photoelectrophoretic imaging suspensionapplied between said electrodes to imagewise activating electromagneticradiation to form an image on at least one of said electrodes from saidsuspension exposed to radiation and subjected to an electric fieldestablished between the electrodes, and

means to effect a heat transfer with said thermoconductive layer tolower its resistivity for dissipating accumulated charge thereon toprepare said apparatus for reuse.

